Hearing aids are electronic instruments worn in or around the ear that compensate for hearing losses by amplifying sound. Because hearing loss in most patients occurs non-uniformly over the audio frequency range, most commonly in the high frequency range, hearing aids are usually designed to compensate for the hearing deficit by amplifying received sound in a frequency-specific manner. Adjusting a hearing aid's frequency specific amplification characteristics to achieve a desired optimal target response for an individual patient is referred to as fitting the hearing aid. The optimal target response of the hearing aid is determined by testing the patient with a series of audio tones at different frequencies. The volume of each tone is then adjusted to a threshold level at which it is barely perceived by the patient. The hearing deficit at each tested frequency can be quantified in terms of the gain required to bring the patients hearing threshold to a normal value. For example, if the normal hearing threshold for a particular frequency is 40 dB, and the patient's hearing threshold is 47 dB, 7 dB of amplification gain by the hearing aid at that frequency results in optimal compensation.
Most often, a new hearing aid user is not fitted with the optimal target response at the first audiologist visit. This is because a patient with a hearing deficit that is suddenly compensated at an optimal level may find the new sounds uncomfortable or even intolerable until adaptation occurs. Patients initially fitted with optimal compensation may even discontinue using their hearing aid. Therefore, it is common practice for the audiologist to initially fit the hearing aid with a sub-optimal degree of compensation which is then ramped up to the optimal level during subsequent fittings at a rate the patient finds comfortable.